1. Field of the Invention
The present invention relates to an acoustic circuit for correcting a frequency characteristic and a phase characteristic of regenerated sound to an optimal state so that the regenerated sound is obtained in a state extremely near to original sound in a sound field for regeneration.
2. Description of the Prior Art
Recently, an amplifier called an audio amplifier capable of regenerating sound with high fidelity is generally used.
The audio amplifier is required to regenerate sound with high fidelity and accordingly a frequency characteristic thereof is adapted to be flat over as a wide frequency range as possible.
A frequency characteristic of a common audio amplifier is substantially flat from about 30 Hz to about 10 KHz while the frequency characteristic is lower with decrease or increase of the frequency in a frequency range other than the above frequency range. Further, the audio amplifier has a phase characteristic that a phase is gradually delayed with increase of the frequency.
In an audio amplifier, heretofore, the quality of regenerated sound has been improved. The improvement of the sound quality is achieved by an equalizer amplifier or the like to attain a flat frequency characteristic in a sound field. However, it has been found recently that it is desirable that a phase characteristic be flat over the entire frequency range as well as the flat frequency characteristic in other to realize higher quality sound.
However, heretofore, an improvement for the phase characteristic which is corrected to be flat over the all frequency range in the audio amplifier is not quite made and accordingly the conventional audio amplifier has a limit to improvement of the quality of regenerated sound. Further, when an amplification in a ultra-low or ultra-high frequency range is to be made in a relation to phase shift, an oscillation may occur in the ultra-low or ultra-high frequency range. Even in this point, there is a limit to improvement of the quality of regenerated sound.
On the other hand, an audio amplifier for an automobile having a room forming a sound field includes an equalizer circuit capable of correcting the sound field characteristic peculiar to the room of the automobile and correcting the sound quality and a frequency characteristic of regenerated sound so that the regenerated sound by the amplifier is obtained in the state near to original sound so that music can be heard in a relatively good condition even in the room of the automobile where the acoustic conditions are not so good.
However, with such a conventional equalizer circuit, since the frequency characteristic can be made flat but the phase characteristic is not taken into consideration, the regenerated sound is very unsatisfactory.
More particularly, in the equalizer circuit, when a single sound of a sine wave is employed and its frequency is gradually increased or decreased so that a phase characteristic of the equalizer circuit is measured by a measuring apparatus, there can be obtained a phase characteristic having no variation over substantially all frequency range apparently. However, since the phase characteristic is obtained as a result of a combination of phase characteristics of a low pass filter, a band pass filter and a high pass filter, a phase distortion occurs due to a phase difference in a frequency range in which phases of each of the filters are overlapped to thereby cause deterioration and impurity of the quality of sound and indefiniteness of orientation of the acoustic sound. The phase distortion is particularly large at an overlap point of the frequency characteristic curves of the filters, that is, in the vicinity of a dip point. This is caused by the fact that the equalizer circuit includes a feedback circuit.
As described above, the improvement of the sound quality in the prior art resides in the realization of the flat frequency characteristic in the sound field. However, when higher sound quality is required, the phase distortion can not be neglected as described above.
The phase of regenerated sound is an important improvement item for eliminating the phase distortion since it is an important element to the orientation of the acoustic image when excellent sound in the psychological aspect which is not measured by any measuring apparatus, directly speaking, natural sound, such as sound having presence or transparency, or sound which does not give fatigue even if listening to the sound for a long time is pursued.